By employing high-throughput sequencing and other modern molecular methods, the profiling of the microbial community with subsequent detailed determination of the vaginal microbiome was finally possible.

The vaginal microbiome refers to the bacterial communities present in the female reproductive tract; a healthy vaginal microbiome is characterized by resilience and stability and resilience, whereas an unstable microbiome could be prone to invasion by pathogenic bacteria and dysbiosis.

Furthermore, specific disturbances of the vaginal microbiome have been involved in complications of pregnancy. Generally, Lactobacillus species are seen as a stability factor, since they are associated with vaginal health and better fertility outcomes. This is the reason why they were promoted as a way to re-establish a healthy vaginal microbiome in order to promote fertility.

Studies that have observed vaginal microbiome have been quite helpful for diagnosing abnormal states, even achieving prediction of outcomes when pursuing assisted reproduction. Current data suggest that the state of vaginal microbiome at the moment of embryo transfer may affect pregnancy outcome.

Stability of Microbiome During Pregnancy

Different research groups have analyzed vaginal microbiome during pregnancy, usually by using samples taken from the vaginal wall epithelium, posterior vaginal fornix or vaginal fluid during gestation. Overall data points to a preponderance of Lactobacillus acidophilusand Lactobacillus gasseri, while Lactobacillus iners, Lactobacillus crispatus or Lactobacillus jensenii are observed less frequently.

There are a plethora of influences on vaginal microbiome stability during pregnancy, such as the absence of menstrual flow, no cyclic hormonal alterations, the absence of vaginal and/or cervical secretions that are associated with the usual reproductive cycle, but also altered sexual activity during pregnancy. Most of these factors contribute to the resilience of the vaginal microbiome.
 

Vaginal microbiome dynamics is significantly different when ethnicity is considered. For example, Caucasians have decreased variation of the microbiota during the progression of pregnancy between different subjects, while African-American women show quite more divergence between different individuals.

Regardless of ethnicity, the vaginal microbiome just after birth dramatically changes and becomes less Lactobacillus-dominated. There is also greater biodiversity of species, irrespective of the original microbial make-up.

Vaginal Microbiome and Preterm Birth

According to the World Health Organization (WHO), preterm birth can be defined as delivery before 37 weeks of pregnancy are completed. In 2012, WHO reported that incidents of preterm birth represent a leading cause of newborn death, as well as a major cause of behavior and neurodevelopmental disorders around the world.

Vaginal microbiome and specific species of Lactobacillus that found their niche in the vaginal tract play a considerable role in preterm birth risk. Although the concentration of lactobacilli and preterm birth are not correlated, vaginal microbiomes of women that experienced preterm birth are shown to be dominated by a specific species – Lactobacillus crispatus.

Moreover, there are significant differences in average microbial diversity between women who experienced a preterm delivery and those who successfully carried to term, at least in the Caucasian race. Among women that already experienced preterm delivery, higher levels of Leptotrichia, Sneathia and Mobiluncus species before 16 weeks gestation may predispose them to this incident again.

What is interesting to note is that none of the pregnant women that experienced preterm birth harbored Atopobium. Studies that link specific bacteria with this event are increasingly being published (thus far there is good data focusing on Ureaplasma and Bifidobacterium), but further research approaches are warranted.

All of this means that probiotics may be an effective approach to address dysbiotic states affecting fertility and pregnancy outcomes. They hold the promise of restoring the ecological homeostasis of the genital tract, although our knowledge of healthy microbiome in pregnancy is still in its infancy.